The present invention is directed, in general, to a method of making prescription eyeglasses, and more particularly to a method of shaping or edging prescription lenses to a desired shape to fit selected eyeglass frames.
Because eyeglass frame styles have become as much a matter of fashion as of function, and because of the rapid changes that occur in the world of fashion, the eyeglass frame industry has developed to the point where it produces thousands of new styles of frames every year, with each style being made in several sizes. Although a particular frame may typically stay in fashion for six months or so, the various styles tend to accumulate in stock and in the public domain for many years. Thus, there may be hundreds of thousands of different styles and sizes of eyeglass frames in use or available in the trade at any one time. To fit a particular eyeglass frame with a prescription or even a nonprescription lens, a retail optical or wholesale laboratory must shape a lens blank to fit the frame selected by the customer.
Lens blanks are generally round, are of a variety of standard diameters, and are ground and polished to the desired prescription. These lens blanks must be accurately edge-ground to the specific shape required by a particular frame. Because the safety of an eyeglass wearer is dependent on a tight precision fit of lens to frame for retention of the lens around its entire periphery, the edging of lenses to a frame is an exacting task. To aid in this, frame manufacturers routinely provide patterns shaped to match the shape of a particular frame, which patterns are then used as cams, or guides, for conventional edge grinders. The pattern is placed in the edge grinder, a size adjustment is made, and as the pattern is rotated, a follower arm traces its perimeter shape, translating that shape into a continously varying radial distance from the center of rotation of the pattern. The coaxial follower retains a lens blank and rotates the lens into a grinding wheel while, at the same time, varying the radial distance between the center of rotation of the lens and the surface of the grinding wheel. The lens rotates with the blank, so that the motion of the lens exactly follows the shape of the pattern and thus a lens blank is ground to the desired size and shape.
To permit widespread marketing of a frame, the pattern for that frame must be supplied to all the eyeglass retailers or wholesale eyeglass laboratories which might be required to produce a lens for that particular frame. Thus, for each new frame style, a manufacturer must make and distribute up to 40,000 patterns, depending on the number of retailers and laboratories in its marketing area. With 13,000 new styles and sizes available each year, and with each style requiring up to 40,000 patterns to be distributed, a staggering number of patterns may be required each year, with an annual unreimbursed cost to the frame manufacturers in excess of several million dollars per year.
These large numbers of patterns are not only a problem for frame manufacturers, but for retail and wholesale manufacturers as well, for it is the burden of retailers and wholesale laboratories to maintain an accurate inventory of these patterns, keeping track of them so that they can be quickly located when an order is received for a lens to fit a particular frame. Thus, the retailer or wholesaler must go to the expense of providing internal procedures to insure that patterns are available, and that a pattern is replaced in inventory immediately after use so that it can be found the next time. A large amount of storage space is required for all of these patterns, adding to the cost and creating an impossible task not only for small retailers, but for wholesale laboratories as well. The large numbers of patterns, their small size, and their similarities in shape all lead to severe problems in handling and lead to costly errors or delays in the production of lenses. In large pattern libraries, costs become prohibitive, forcing stores and laboratories to limit the availability of frame styles to only a small fraction of those currently available which, in turn, can lead to lost customers.
Retail stores or wholesale laboratories must maintain elaborate inventory systems which are labor-intensive and costly, and when, in spite of this, a pattern is lost, they must then face the choice of either losing the customer or making a pattern manually by tracing the frame and manually fabricating a pattern. This requires an investment in expensive machinery, leads to inaccuracies in the patterns, resultant errors in the lens grinding, requires extra labor, increases the cost of the lens and results in delays. In the case of partial or wholly rimless frames, where there is little or no frame to trace, the loss of a pattern means that the retailer must go back to the manufacturer for the pattern, leading to further delays in supplying the completed prescription glasses. Thus, the present system of pattern inventory is wholly unsatisfactory.
A further problem encountered in the process of producing prescription eyeglasses is that when a lens blank is being edge-ground to fit a particular frame, it is necessary accurately to position the optical center of the lens with respect to the frame so that the optical center for each lens in the frame coincides with the wearer's pupils. This requires a specific fitting of the frame and its lenses to the patient. The difficulty is that the optical center for a lens blank usually does not coincide with its geometric center after the lens has been ground and polished to prescription. Further, the frame manufacturer has no idea where the optical center must be located within a particular frame, since the manufacturer does not know who will be wearing that frame, and for this reason it is standard practice for the frame manufacturer to provide lens grinding patterns which use the "box" geometric centers of frames as their points of reference. A lens blank, on the other hand, is usually marked to identify its optical center, but not the frame geometric center; therefore, each time a lens is to be edged, it is necessary to calculate the location of its frame geometric center with respect to its optical center and to then mount, or block, the lens blank at its frame geometric center for edging under the control of the pattern. The location of the frame geometric center of the lens with respect to the lens optical center differs for each frame and each wearer and thus must be individually calculated using standard layout equipment in a separate manufacturing step. Such calculations are exceptionally subject to human error which can easily result in a pair of lenses being edged incorrectly. Since the margin of profit is very small in this field, particularly at the wholesale laboratory level, an error in one pair of lenses can result in a loss which offsets the profits from many correctly-ground pairs of lenses.
More particularly, under present practice, a lens blank is ground and polished to its prescription powers by first connecting a support block to the lens workpiece, then mounting it in grinding and polishing machinary to produce the desired prescription optics. The support block is then removed and the lens blank is tested to measure its compound optical powers, locate its cylinder axis and locate its optical center which is then marked on the lens blank. When the lens blank is to be edge ground and mounted in a frame, the marked optical center may not coincide with the pattern's geometric center, so it is necessary to locate the geometric center of the lens in order to cause the lens to rotate about the same axis as the rotational center of the frame and frame pattern, when the lens is remounted in the edger machinery. When all of this is done properly, the optical center of the lens will fall in the correct location for the patient within the frame when it is mounted. This location of the geometric center of the lens blank is accomplished on a "layout" blocking machine which is operated by hand to locate the geometric center with respect to the previously located optical center of the lens. The proper geometric center becomes the point to which a mounting block is secured to the lens, by which the lens may be mounted in the edge grinding machinery. This hand "layout" operation is a significant source of error in grinding lenses.
A further source of error in the process is the fact that a pattern usually is not formed in a one-to-one size ratio to the lens to be ground. Frames of the same shape generally are furnished in different sizes, but only a single pattern size is provided for each style. The edge grinding machine must be adjusted to grind a lens larger or smaller than the pattern, but in the same geometric shape. However, this means that a particular pattern cannot accurately be compared to the shape of a particular frame before the lens is ground, and thus it is possible to select the wrong pattern for a particular frame and have no way of knowing it until after the lens is ground incorrectly. Furthermore, since the size ratio must be set by hand on the machine, this is another operation which is very prone to human error, leading to additional lens spoilage and slowing down the processing of a prescription.
From the foregoing, it is seen that the present procedures for producing prescription lenses is inefficient, expensive, subject to error and delay, and thus is generally unsatisfactory.